1. Technical Field
This invention relates generally to heating appliances such as storage-tank type hot water heaters with tank diameters between 12 to 26 inches and, more particularly, to the disposition of the inlet dip tube.
2. Background Art
In the prior art, a storage-tank water heater replaces hot water withdrawn from the top of the tank with cold water delivered to the bottom of the tank. Because typical tank heating elements cannot heat the water as fast as it is withdrawn, cold water will eventually fill the tank. Even before the tank is filled with cold water, the incoming cold water mixes freely with the heated standing water in the tank thereby causing deterioration of the tank's water temperature. This mixing is partially the result of the convection currents established within the tank. The storage tank in these heaters generally has a diameter between 12 and 26 inches.
Because of this mixing, hot water delivered by a typical hot water heater will gradually decrease in temperature while water is being withdrawn, only a small amount of high temperature water is delivered relative to the tanks total capacity. The hot water delivered to the outlet above a specified temperature can obviously be extended by increasing the size of the tank or by increasing the BTU input of the heating elements or gas/oil burner. The temperature of hot water at the outlet can also be maintained by preventing the mixing of hot and cold water within the tank.
Attempts have been made in the past to contain and control the mixing of hot and cold water by providing separate chambers within the tank for cold and hot water. Miller U.S. Pat. Nos. 2,833,273 and 3,244,166 employ separate chambers within the tank at the inlet. Gulick U.S. Pat. No. 2,207,057 uses a small baffle over the inlet to control mixing. Fox U.S. Pat. No. 787,909 shows the use of a movable barrier. In substantially different constructions employing the concept of compartmentalization, Jacoby U.S. Pat. No. 2,625,138 divides the tank into a plurality of separate vertical layers by using numerous horizontal baffles and Pruitt U.S. Pat. No. 2,311,469 shows a burner in which several secondary combustion chambers stratify the water in the storage tank.
While these prior art designs tried to reduce flow created by the usual high velocity of incoming cold water and tried to separate hot and cold water layers, none have taken note of the existence of possible convection currents and, thus, none limit the formation of these thermal currents in the tank and preserve the smooth horizontal boundry layer between hot and cold water within the tank. Further, these convection thermal currents are believed to flow primarily along the smooth side surfaces of the tank and are enhanced by the smooth inner surface of the curved top, the "domed" top being common in pressure tanks because of their structural strength. These closed loop currents greatly enhance the mixing of hot and cold water. My U.S. Pat. Nos. 4,632,065 and 4,739,728 attempt to stop mixing caused by these convection currents.
In these patents the mixing is prevented in a more active manner by presenting a physical obstruction to the convection currents. It requires new components to be added to the tank construction. Whereas, the invention disclosed herein prevents mixing in a more passive manner by simply rearranging the same components. This is novel, unique and very cost effective.
In the earlier storage-tank type water heater designs, the incoming cold water was introduced in the bottom of the tank through a side inlet also located adjacent to the bottom of the tank. This was done to maintain maximum physical separation between the incoming cold water and the heated water. Later, for the sake of installation convenience, the side inlet was replaced by a top inlet. The top inlet allows installation in more confined spaces, because it provides easier access to pipe connections. However, to prevent the ready and spontaneous mixing that will occur with the top inlet, a thermoplastic dip tube is used which introduces water into the tank in approximately the same area as before, namely, at the bottom of the tank. The location of the dip tube was always and of necessity off-center, because the exhaust gas flue occupied the central location in gas and oil heaters and the heating elements occupied this space in electric water heaters. At present, the dip tube is spaced 4 inches from the central vertical axis. These dip tubes are made of plastic material which is susceptible to deformation and other damage under high temperature conditions that can occur under a "dry heat" condition. "Dry heat" damage occurs when the dip tube is not submerged in water as it would be under normal conditions. A "dry heat" condition can occur when the heater is inadvertently turned on without first completely filling the tank with water. In addition to the problem posed by the flue or the heating elements occupying the central space, to avoid this problem, the dip tube was always placed off-center to maintain a physical distance from the centrally located heating elements. However, in an off-center location, the dip tube does not effectively counter uprising convection currents since they are located at the center of the tank.
There is another problem commonly associated with gas and oil water heaters. Frequent withdrawals of small quantities of hot water causes frequent firing of the gas/oil burner. With each firing of the burner, the temperature of the water overshoots the thermostat setting. This overheated water rises and accumulates at the top. This phenonmenon is sometimes referred to as "stacking". Since the thermostat is located in the bottom of the heater, the temperature of the water in the top can become dangerously high. To alleviate this problem, present water heaters use dip tubes which terminate above the bottom of the tank about 1/3rd of the way up. Some heaters use a dip tube design which partially introduces incoming cold water in the upper portion of the tank to moderate the excessively high water temperature. To accomplish this, the dip tube is provided with a hole which opens or closes in response to the temperature of the surrounding water. Both these solutions have an adverse effect on the recovery rating of the heater. The invention disclosed herein can provide a solution to the "stacking" problem without adversely effecting the recovery rate.